This invention relates to a thermographic stencil sheet of the type which includes an ink-impervious coating of a heat-flowable composition on an ink-pervious base sheet, and to a method of making an imaged stencil sheet therewith by subjecting image areas of the stencil sheet to heat generated by infrared ray absorption.
A thermographic stencil sheet including an ink-pervious fibrous base sheet and an ink-impervious coating thereon of a heat-flowable composition of thermoplastic film-forming material comprising a cellulose organic ester, and plasticizing material partially but incompletely compatible with the film-forming material is disclosed in various embodiments in U.S. Pat. Nos. 3,694,244, 3,694,245, 3,704,155, 3,824,116 and 3,824,117. The stencil sheet now is in widespread commercial use. It is employed preferably in a stencil sheet assembly including a contacting receiving or absorbent sheet on one surface thereof, and a more rigid backing sheet on the opposite surface thereof and to which the receiving sheet and the stencil sheet are mounted. In use, an original, such as a typed or printed sheet, is inserted between the stencil sheet and the backing sheet, and the assembly is exposed to infrared radiation on the face side of the receiving sheet in a thermocopier such as a Weber Thermal Imager (Weber Marking Systems) or a Thermo-Fax machine (3M Company). Heat is generated in the radiation absorptive graphic portions of the original to cause the stencil sheet composition to flow in corresponding areas and thereby produce corresponding image openings in the stencil sheet. A portion of the composition rendered flowable adheres to and/or is absorbed by the receiving sheet and/or adjoining areas of the stencil sheet. The original and the receiving sheet are separated from the imaged stencil sheet, the stencil sheet and the backing sheet are placed on a mimeograph duplicating machine followed by a separation of the backing sheet, and the machine is operated to produce multiple mimeograph copies of the original.
The improvements provided by the several patents have provided stencil sheets which produce good quality prints, suitable for various uses. However, there remains room for improvement, especially in providing for the variables of materials, procedures and machines encountered in use, which variables may affect copy quality and ease and reliability of operation. While imaging takes place relatively rapidly, advantages would be achieved by further increasing the speed of imaging. It will also be advantageous to increase the latitude of the imaging speed, so as to produce good stencils even though the machine settings may not be optimum.